The invention relates to a method for controlling steam generators of pressurized water nuclear reactors (PWRs) and more particularly to method for controlling the flow of feedwater to the secondary sides of PWR steam generators.
In commercial PWRs utilized to generate electrical power, reactor coolant water (or primary water) recirculates between a reactor pressure vessel and one of a plurality of in-parallel steam generators in a closed loop known as a reactor coolant system (or a primary system). In a steam generator, the heat in the recirculating primary water flowing through the primary side (i.e., the tube side) passes through the walls of the tubes and is absorbed by relatively cool secondary water flowing on the secondary side (or shell side). The transferred heat generates steam on the secondary side at a temperature of about 500° F. or more and at a pressure of about 800 psi or more. The steam flows out of the steam generators to turbines that generate the electrical power. The exhaust steam from the turbines is condensed and recirculated to the steam generators as feedwater. The nominal flow of feedwater to a steam generator of commercial PWRs may be 100,000 gpm or more during normal power generation operations.
U.S. Pat. Nos. 6,021,169; 5,455,763; 5,192,493; 4,777,009 and 4,728,481, which are incorporated by this reference, disclose various control systems for controlling steam generators during power operations. Such control systems generally have two mode proportional-integral controllers with feedback for providing demand signals to positioners operating flow control valves and/or speed controllers controlling the main feedwater pumps. Proportional (gain) mode generally shapes the response curves, with higher gains generally giving faster transients but more oscillatory responses. Integral (reset) mode eliminates steady state offsets. Proportional-integral controllers may also have a derivative (rate) element that allows higher proportional gains for high ordered systems.
These PWR steam generator control systems (i.e., systems using proportional-integral control, especially with a derivative element) have a tendency of continuously “hunt” to  minimize steady state errors, which is a principal objective of control engineers. Advantageously, proportional-integral control systems can be readily analyzed. Undesirably, this “hunting” tendency causes accelerated wear of various control hardware such as valve stems, positioners and actuators.
Filtering networks known as “deadbands” have long been employed to reduce “hunting” and consequent hardware wear by control engineers in applications where discrete variables are monitored. Thus, deadbands have been employed to control the movement of control rods in reactor pressure vessels which move in discrete steps. See, e.g., U.S. Pat. No. 4,707,324. However, because of the analytical complexity of analyzing control systems employing proportional-integral systems with deadbands in addition to the complication of designing the necessary hardware, the nuclear industry has been unwilling to employ deadbands with the proportional-integral (with or without a derivative element) controllers employed to continuously control steam generators.